In a phased array ultrasound imaging system, an ultrasound transducer includes an array of transducer elements. The system includes a multiple channel transmitter and a multiple channel receiver connected to the transducer. Each transmitter channel causes a selected transducer array element to transmit an ultrasound pulse into an object being imaged, typically the human body. The transmitted ultrasound energy is steered and focused by applying appropriate delays to the pulses transmitted from each transducer array element, so that the transmitted energy adds constructively at a desired focal point. The ultrasound energy is partially reflected back to the transducer array by various structures and tissues in the body.
Steering and focusing of the received ultrasound energy are effected in a reverse manner. The reflected ultrasound energy from an object or structure arrives at the array elements at different times. The received signals are amplified and delayed in separate receiver channels and then summed in a receive beamformer to form a receive beam. The delay for each channel is selected such that the receive beam is steered at a desired angle and focused at a desired depth. The delays may be varied dynamically so as to focus the beam at progressively increasing depths along a scan line as the ultrasound energy is received. The transmitted beam is scanned over a region of the body, and the signals generated by the beamformer are processed to produce an image of the region.
In order to obtain the highest quality image, both the transmit beam and the receive beam should be focused at each point in the area being imaged. However, the time required to obtain an image in this manner would be prohibitive. In most prior art systems, the transmit beam is typically focused at a single focal depth, and the receive beam is dynamically focused only in azimuth, the direction perpendicular to the divisions between elements of the transducer array. For both transmit and receive beams, the elevation focus is established by an ultrasound lens mounted on the transducer. As a result, the transmit beam is out of focus at points displaced from the transmit focal point, and the receive beam is out of focus in the elevation direction except at a fixed focal point. These factors cause portions of the image displaced from the focal points to be degraded in quality. U.S. Pat. Nos. 4,359,767 and 4,670,683 disclose ultrasound transducers having rows and columns of transducer elements. The disclosed transducer configurations are stated to permit elevation focusing at different depths.
U.S. application Ser. No. 08/006,084, filed Jan. 19, 1993 and assigned to the assignee of the present application, discloses an ultrasound transducer having rows and columns of transducer elements. Two or more ultrasound pulses are transmitted at the same steering angle, but at different focal depths. The received signals in the region of each transmit focal point are "spliced" together to form a single receive line at each steering angle. The transmitted pulses are focused both in elevation and azimuth by energizing different transducer elements at different focal depths.